Method and apparatus for improving the accuracy with which the speed of a fluid is measured

ABSTRACT

The present invention relates to a method and associated apparatus for improving the accuracy with which the speed of a fluid, such as a liquid, in particular blood flowing in a duct, such as a blood vessel, in particular the aorta, is measured by means of a signal emitted by a Doppler transducer ( 4 ). In characteristic manner, according to the method, the Doppler transducer is associated with a programmable memory ( 50 ) which contains at least one correction data item for correcting the Doppler signal transmitted by the transducer ( 4 ) to a transducer control and computer unit ( 8 ). Said computer unit ( 8 ) incorporates said signal correction data item in its computation (at  16 ) of each speed measurement on the basis of each signal emitted by the Doppler transducer, and it computes the speed value while taking account of said correction data item so as to provide a corrected measurement of the speed of said fluid, thereby improving its accuracy.

This application is a continuation of Ser. No. 09/922,603 filed Aug. 3,2001 and now U.S. Pat. No. 6,506,159, which is a continuation of Ser.No. 09/522,089 filed Mar. 10, 2000 and now U.S. Pat. No. 6,287,260.

The present invention relates essentially to a method and to apparatusfor improving the accuracy with which the speed of a fluid, such as aliquid, in particular blood flowing in a duct, such as a blood vessel,in particular the aorta, is measured by means of a signal emitted by aDoppler transducer.

BACKGROUND OF THE INVENTION

Document FR-A-2 424 733 INSERM discloses an ultrasound intracorporealprobe that is inserted into the esophagus to measure the flow rate inthe aorta. That prior probe is characterized by a catheter structurewhose distal portion has a bag that can be inflated from the outsidewith a liquid and that surrounds the ultrasound emitter which is housedentirely inside the bag which serves to prevent the emitter from movinginside the duct and which serves to couple the emitter acoustically. Theemitter is mounted to rotate inside said inflatable bag on a supportblock which is disposed substantially on the longitudinal axis of theprobe, and it is rotated by a flexible cable connected at its proximalend at the outside to rotary drive means, e.g. in the form of a knob(see the claims and the corresponding text describing the figures, inparticular page 2, line 24 to page 4, line 29).

That prior INSERM document has been improved in the context of documentU.S. Pat. No. 5,479,928 according to which the intracorporeal probe hasin combination: at least one broad-beam ultrasound transducer fixed onthe support block in such a manner as to be oriented at an angle ofinclination that is not perpendicular relative to the longitudinal axisof the probe; and at least one narrow beam ultrasound transducer fixedon the support block so as to be oriented at an angle that isessentially perpendicular relative to the longitudinal axis of the probeso as to be oriented substantially perpendicularly relative to thelongitudinal axis of a duct in which the speed of a liquid is to bemeasured, and in particular the flow rate of the liquid, specificallythe flow rate of blood when the duct is a blood vessel.

The improvement according to that document is entirely satisfactory andis available on the market from SOMETEC under the trade name DYNEMO3000®.

OBJECTS AND SUMMARY OF THE INVENTION

A main object of the present invention is to resolve the novel technicalproblem consisting in supplying a solution enabling account to be takenof each feature of the Doppler transducer in order to improve theaccuracy with which the speed of a fluid, such as a liquid, is measuredby means of a signal emitted by such a Doppler transducer.

Another main object of the present invention is to supply a solutionmaking it possible also to take account of the 3D position of a Dopplertransducer, and in particular the angle at which it emits the ultrasoundbeam, thereby improving the accuracy with which the speed of a fluid,such as a liquid, is measured by means of the signal emitted by such aDoppler transducer.

Another main object of the present invention is to resolve the saidnovel technical problems in a manner that is particularly simple, lowcost, reliable, and usable on a medical and industrial scale.

Those problems are resolved for the first time by the present inventionat low manufacturing cost by means of a design that is particularlysimple, using a small number of parts, while conserving the operatingadvantages of prior art probes, in particular the improved probeconstituting the subject matter of document U.S. Pat. No. 5,479,928, andsold in the form of the DYNEMO 3000® appliance.

In a first aspect, the present invention provides a method of improvingthe accuracy with which the speed of a fluid, such as a liquid, inparticular blood flowing in a duct, such as a blood vessel, inparticular the aorta, is measured by means of a signal emitted by aDoppler transducer, the method being characterized in that the Dopplertransducer is associated with a programmable memory which contains atleast one correction data item for correcting the Doppler signaltransmitted by the transducer to a transducer control and computer unit,in that said computer unit incorporates said signal correction data itemin its computation during each speed measurement on the basis of eachsignal emitted by the Doppler transducer, and computes the speed valuewhile taking account of said correction data item, so as to provide acorrected measurement of the speed of said fluid, thereby improving theaccuracy of the measurement.

According to an advantageous characteristic of the method, it ischaracterized in that said Doppler transducer is incorporated orintegrated in a probe, in particular an intracorporeal Doppler effectprobe, said Doppler transducer being mounted on the probe to emit anultrasound beam at an angle relative to the longitudinal axis of theprobe; and in that said probe also comprises said programmable memory.

According to another advantageous implementation of the method, it ischaracterized in that said programmable memory also contains at leastone sensitivity data item for informing the user of loss of sensitivityto the Doppler signal, and in that said transducer control and computerunit verifies said sensitivity data item present in the programmablememory on each measurement of the signal transmitted by the transducerin order to verify that the sensitivity as actually obtained on a signaltransmitted by the transducer is not too far removed from thesensitivity value present in the programmable memory, and on goingbeyond a specified limit value, said transducer control and computerunit issues a signal to the user indicative of a loss of sensitivity.

According to yet another advantageous characteristic of the invention,the method is characterized in that the said signal correction data itemis obtained on the basis of tests, preferably performed at themanufacturing site, while performing preliminary use tests on theDoppler transducer in order to verify the reliability of its signal.

According to another advantageous characteristic of the method of theinvention, it is characterized in that the sensitivity data item isobtained during tests, preferably performed at the manufacturing site,while measuring the flow speed of a fluid that is flowing at a knownspeed.

According to yet another advantageous characteristic of the method ofthe invention, the method is characterized in that the signal correctiondata item comprises at least the angle at which the Doppler beam isemitted by the Doppler transducer relative to the axis of the probe, sothat the speed value takes account of said real working angle of thebeam from the Doppler transducer.

Advantageously, the sensitivity data comprises at least one average of aplurality of sensitivity measurements obtained with a correspondingnumber of uses of the Doppler transducer, each sensitivity measurementresulting from the amplitude of the signal received from the transducer.

According to another advantageous characteristic of the method of theinvention, it is characterized in that the transducer control andcomputer unit continuously computes the mean of a plurality of recentlycalculated sensitivity measurements and compares it with the sensitivitymean initially written as sensitivity data in the programmable memory,and, beyond a certain difference relative to the initially programmedsensitivity measurement, issues a signal to the user indicative of aloss of sensitivity.

According to yet another advantageous characteristic of the invention,the method is characterized in that when the Doppler transducer operatesin combination with an additional transducer, e.g. for measuring thediameter of a duct in which said fluid flows, at least one sensitivitydata item relating to said additional transducer is preferably alsoprovided in said programmable memory in order to verify its sensitivityover time and likewise issue a signal to the user in the event ofsensitivity being lost.

In a second aspect, the present invention also provides an apparatus forimproving the accuracy with which the speed of a fluid, such as aliquid, in particular blood flowing in a duct, such as a blood vessel,in particular the aorta, is measured by means of a signal emitted by aDoppler transducer, the apparatus being characterized in that itcomprises a programmable memory containing at least one correction dataitem for correcting the Doppler signal transmitted by the transducer toa transducer control and computer unit, and in that means are providedto enable the computer unit to incorporate said signal correction dataitem in computing each speed measurement on the basis of each signalemitted by the Doppler transducer and to compute the speed value takingaccount of said correction data item so as to provide a correctedmeasurement of the speed of said fluid, thereby increasing its accuracy.

In an advantageous embodiment, said Doppler transducer is incorporatedor integrated in a probe, in particular in a Doppler effectintracorporeal probe, said Doppler transducer being mounted on the probeto emit its ultrasound beam at an angle relative to the longitudinalaxis of the probe; and said probe also comprises said programmablememory connected to said control and computer unit, which memory is thussecured to the probe and is dedicated thereto.

In another advantageous embodiment of the invention, said programmablememory also contains at least one sensitivity data item for informingthe user of a loss of sensitivity in the Doppler signal, and thetransducer control and computer unit verifies said sensitivity data itempresent in the programmable memory on each measurement of the signaltransmitted by the transducer in order to verify that the sensitivityactually obtained on the signal transmitted by the transducer is not toofar removed from the sensitivity value present in the programmablememory; signal-issuing means are provided; and in the event ofsensitivity going beyond a set limit value, the transducer control andcomputer unit issues a signal to the user via said signal-issuing meansto inform the user of a loss of sensitivity.

In another advantageous embodiment of the invention, said signalcorrection data item is obtained from tests preferably performed at themanufacturing site while performing preliminary use tests on the Dopplertransducer in order to verify the reliability of its signal.

According to another advantageous characteristic of the invention, thesensitivity data item is obtained during tests that are preferablyperformed at the manufacturing site while measuring the flow speed of afluid that is flowing at a known speed.

According to another advantageous characteristic, the signal correctiondata item comprises at least the angle at which the Doppler beam isemitted by the Doppler transducer relative to the axis of the probe, sothat the speed value takes account of said real working angle of thebeam from the Doppler transducer as actually mounted on the probe.

According to another advantageous characteristic of the invention, thesensitivity data item comprises at least an average of a plurality ofsensitivity measurements obtained over a corresponding number of uses ofthe Doppler transducer, each sensitivity measurement resulting from theamplitude of the signal received from the transducer.

According to another advantageous characteristic of the invention, thetransducer control and computer unit continuously computes the mean of aplurality of recently calculated sensitivity measurements and comparesit with the sensitivity mean initially entered as sensitivity data intothe programmable memory and beyond a certain difference relative to theinitially programmed sensitivity measurement, issues a signal to theuser via said signal-issuing means to indicate a loss of sensitivity.

According to another advantageous characteristic of the invention, saidapparatus further comprises an additional transducer operating incombination with the Doppler transducer, said additional transducerbeing intended, for example, to measure the diameter of a duct in whichsaid fluid flows, said programmable memory preferably further containingat least one sensitivity data item concerning said additional transducerso as to verify its sensitivity over time and likewise issue, via saidsignal-issuing means, a signal to the user in the event of a loss ofsensitivity.

It will thus be understood that by means of the invention all of thepreviously-mentioned advantages are obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, characteristics, and advantages of the invention appearclearly from a presently preferred embodiment of the invention givenmerely by way of example and which does not limit the scope of theinvention in any way. In the drawings:

FIG. 1 shows a probe comprising a Doppler transducer in accordance withFIG. 1 of U.S. Pat. No. 5,479,928, but modified to incorporate aprogrammable memory in accordance with the present invention, the probebeing shown in cross-section and in elevation, and, as in FIG. 1 of U.S.Pat. No. 5,479,928, in its working position in the esophagus facing ablood vessel, in this case the aorta 10;

FIG. 2 is a diagram showing the operation of calibrating the FIG. 1probe including the programmable memory in accordance with the inventionso as to determine the characteristic of the Doppler transducer actuallymounted on the probe; and

FIG. 3 is a calibration curve obtained during tests performed at themanufacturing site to calibrate the Doppler transducer 4 using theequipment of FIG. 2.

PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 reproduces FIG. 1 of U.S. Pat. No. 5,479,928=FR-A-2 695999=EP-A-0 595 666 and uses essentially the same reference numerals.With reference to FIG. 1, a catheter-shaped probe 1 for measuring thespeed of flow of a fluid F is manufactured in conventional manner. Forexample, the probe is preferably intended to measure the speed of flowof blood in the aorta 10, with the probe comprising a Doppler transducer4 having a broad beam 4 a and being, by way of example, of the same typeas that described in U.S. Pat. No. 5,479,928=FR-A-2 695 999=EP-A-0 595666, to which the person skilled in the art can refer.

In this preferred embodiment, the Doppler transducer 4 is designed onmanufacture to present an angle of inclination for its ultrasound beamof 60° relative to the longitudinal axis x-x′ of the probe 1.

Incidentally, and preferably, the Doppler transducer 4 operates incombination with an additional transducer 5, for example and preferablyin the context of measuring the flow speed of blood in a blood vessel,in this case the aorta 10, a transducer which produces a narrow beam 5 aas described in document U.S. Pat. No. 5,479,928, with the transducerbeing placed parallel to the longitudinal axis of the probe so that itsultrasound beam extends perpendicularly to the longitudinal axis of theprobe for the purpose of measuring the diameter and thus the flowsection S of the duct 10 in which there flows the fluid F whose speed isto be measured, as described in the above-specified documents and as isalso known to the person skilled in the art, in particular from thosedocuments.

The other references in FIG. 1 which are identical to those of FIG. 1 inU.S. Pat. No. 5,479,928=FR-A-2 695 999=EP-A-0 595 666 have the followingmeanings: reference numeral 1 represents the outside portion of thecatheter-shaped probe which, when installed in a duct, in this case theesophagus 13 facing a blood vessel, in this case the aorta 10, will takeup a position that is fixed once the inflatable balloon 6 has beeninflated in the manner known to the person skilled in the art.

The catheter-forming probe 1 has an internal flexible cable 2 connectedat one of its ends to the support block 3 on which the ultrasoundtransducers 4 and 5 are mounted. The transducers 4 and 5 are connectedto an electric cable 7 placed in the probe 1 at its distal end andleaving the probe 1 on the outside for connection to a computer centeror unit for controlling the transducers and for processing the signalsthey deliver. The external end of the flexible cable 2, remote from itsend connected to the support block 3, is connected to a drive member 9,in this case modified in accordance with the present invention to be inthe form of a handle suitable for being taken hold of to rotate theflexible cable 2 appropriately about its own axis so as to rotate thesupport block 3, thereby directly turning the ultrasound beams 4 a and 5a respectively of the transducers 4 and 5 appropriately relative to theduct 10 such as the aorta in which it is desired to measure the flowspeed of the fluid F, in this case blood. The control and computer unit8 comprises, as described in U.S. Pat. No. 5,479,928 and itsequivalents, means 14 connected to the transducer 5 by a link 7 ₁designed to determine the amplitudes of echo signals received by thenarrow beam transducer 5. These determination means 14 are connected tomeans 15 designed to detect the amplitude maxima in the reflectedsignals.

Means 16 are also provided in the control and computer unit 8 todetermine the range P₂-P_(l) circumscribing the section of the duct 10at two opposite extreme points by taking account solely of the echoes ofthe transducer 5 coming from said range P₂-P₁. The means 16 areconnected to the means 15 in order to determine the range d₂-d₁ whichcorresponds to the two extreme points of the duct, in this case theaorta 10, as detected by the amplitude maxima in the reflected echoes ofthe signals from the transducer 5. By knowing the range d₂-d_(l), it ispossible to calculate the flow section S since it is conventional toconsider the duct 10, in this case the aorta, to be circular in section.The means 16 which conventionally comprise a microcomputer withappropriate software also takes account of the angle of inclination θbetween the two beams 4 a and 5 a as shown in FIG. 1 and as known, inparticular by construction. These means 16 control selection means 17connected to the transducer 4 by a link 7 ₂. The means 17 make itpossible to select only those echoes of signals from the transducer 4which are obtained over a response time interval corresponding to therange P₂-P_(l). The selection means 17 are connected to conventionalsignal processing means 19 for obtaining a Doppler signal. Theseprocessing means 19 are also connected to conventional means 20 suitablefor determining the mean speed V_(m) of the fluid F, in this case blood,as averaged over the section S of the duct 10, in this case the aorta.

As described in U.S. Pat. No. 5,479,928, the unit 8 also has means 21suitable for measuring the energy backscattered by moving particles, inthis case red corpuscles in the blood. The output from the measuringmeans 21 is connected to means 22 designed to allow backscattered energyto pass through at one or more defined instants, in particular duringsystole when measuring blood speed, said means 22 also being connectedto means 23 suitable for determining those defined instants, and inparticular the instant at which systole occurs when blood is beingmeasured. The means 22 deliver the value of the backscattered energyE_(S) during systole when measuring blood.

As described in U.S. Pat. No. 5,479,928, outside systole and inparticular during diastole, the area S_(D) covered by the particlesactually in motion is likely to be smaller than the full section S.

By taking account of backscattered energy during systole E_(S) andduring diastole E_(D) it is possible to determine the real flow sectionor the effective ideal section S_(D) involved in the flow rate. Thisarea is defined by the following mathematical equation:S _(D) =S.(E _(D) /E _(S))=S.K

The correction factor K is determined by correction means 24 connectedto the means 21 and 22. The correction means 24 weight the factor K byan empirical correction factor which takes account of the technicalcharacteristics of the transducer 4 in use and of the means 19, inparticular the minimum value of the speeds detected and the passband ofthe emitted Doppler signal. The correction means 24 are connected tomeans 25 that are also connected to the means 20 for determining themean speed over the section. The means 25 make it possible to calculatea corrected mean speed V_(c) using the values for the mean speed overthe area and the correction factor K, and thus to calculate the flowrate of the fluid F, in this case blood, moving through the localizedarea S_(D), given knowledge of the section S of the duct 10, in thiscase the aorta, as measured from distances D₁ and D₂ obtained by thetransducer 5 disposed perpendicular to the duct 10.

In an embodiment, the correction factor K can be represented by thefollowing equation:$K = {\left( \frac{E_{D}}{E_{S}} \right)^{n} \times k}$in which:

K=correction factor;

E_(D)=partial backscattered energy as defined above;

E_(S)=total backscattered energy as defined above;

n is a number constituting another corrector factor; and

k is a correction factor as defined above depending on the technicalcharacteristics of the Doppler transducer 4 and of the means that emit,receive, measure, and process the signals associated with the Dopplereffect transducer 4, including the measurement means 19.

In the context of the present invention, in order to improve theaccuracy with which the speed of the fluid is measured, provision ismade for the apparatus that comprises the probe 1 and its control andcomputer unit 8 to further comprise, in accordance with the presentinvention, a programmable memory 50 which is associated with the Dopplertransducer 4 and which contains at least one data item for correctingthe Doppler signal transmitted from the transducer 4 to the transducercontrol and computer unit 8, and in particular to its computation means16. Such programmable memories are commercially available, e.g. memoriesknown as EEPROMs or memories known as flash memories. The computationmeans 16 conventionally comprise, for example, a computer or amicrocomputer having appropriate software for controlling it. In thiscontext, the computation means 16 also has software incorporating saidsignal correction data item as recorded in the programmable memory 50each time it performs computations on each speed measurement as obtainedby means of the Doppler transducer 4.

According to a preferred characteristic of the invention, this signalcorrection data item comprises at least the angle θ at which theultrasound beam is emitted by the Doppler transducer 4. This angle isdetermined by performing a plurality of speed measurements using theDoppler transducer 4 on a fluid that is flowing along a calibrated ductof known diameter at a known speed of fluid flow which is preferablyadjusted to a different known speed value for each measurement.

As is well known to the person skilled in the art, the flow speed of thefluid as obtained by the Doppler effect is derived from the mathematicalequation:$V \approx {\frac{\Delta\quad F}{F_{emit}} \times \frac{C}{{2 \cdot {Cos}}\quad(\theta)}}$in which:

-   -   ΔF=the frequency difference between reception and emission, as a        result of the Doppler effect;    -   F_(emit)=the frequency at which the ultrasound beam is emitted        by the Doppler transducer;    -   C=the speed of propagation of sound in the medium, e.g. in        blood, equal to 1584 meters per second (m/s); and    -   θ=the angle at which the ultrasound beam is emitted by the        Doppler transducer 4 relative to the longitudinal axis x-x′ of        the probe 1.

As a result, starting from the speed value as actually measured andstarting from an average of a plurality of speed measurements atdifferent flow speeds, the exact value of the angle at which theultrasound beam is emitted by the Doppler transducer is obtained, i.e.as emitted by the probe.

This angle θ is thus incorporated in the programmable memory 50 forsubsequent use by the computer unit 8 in calculating the real speed whenmeasuring the speed of a fluid F flowing along a given duct 10, inparticular and preferably the aorta. Preferably, the programmable memoryis incorporated in or forms an integral portion of the probe 1, thushaving the advantage of constituting a “signature” for the probe.

According to another advantageous characteristics of the invention, theprogrammable memory 50 also includes data concerning the sensitivity ofthe Doppler transducer 4.

This sensitivity data is obtained by programming the transducer controland computer unit 8 which, on each speed measurement, memorizes theamplitude of the signal received by the transducer after a Doppleremission, and takes an average over a plurality of measurements tocalculate a reference sensitivity which is subsequently stored by thecomputer unit 8 in the programmable memory 50 and which is subsequentlyused as an initial reference sensitivity, the control and computer unit8 subsequently and on each measurement recalculating the sensitivity ofthe Doppler transducer 4 and preferably also taking an average over aplurality of measurements, which it compares with the initialsensitivity, such that in the event of too great a difference, e.g.greater than ±10% relative to the initial sensitivity measurement, thecontrol and computer unit 8 issues a signal to the user to inform theuser that there has been a loss of sensitivity. By way of example, thissignal can be an alarm, or a warning lamp, or a message.

For example, it is possible to use the rms value of the receivedelectrical signal. By way of example, this value is about 50 μV for aconventional Doppler transducer 4 having dimensions of 3 mm×4 mm andoperating at about 5 MHz.

When an additional transducer 5 is used, in particular and preferablyfor measuring the diameter of the duct 10 in which the fluid F isflowing, the sensitivity of the additional transducer 5 is measured inthe same manner, and this sensitivity measurement is also put into theprogrammable memory 50 so as to be able to give the user a similarsignal concerning loss of sensitivity for this additional transducer,when appropriate. By way of example, the initial sensitivity value maybe 80 μV for an additional transducer 5 having a diameter of 3 mm andoperating at a frequency of about 10 MHz.

It will thus be understood that the invention makes it possible tomonitor proper operation of the Doppler transducer 4 and possibly alsoof the additional transducer 5 effectively and to inform the user, or tomonitor any other additional transducer that may be present on theprobe.

In the context of the invention, any commercially available programmablememory can be used. Examples of programmable memories that are presentlycommercially available are electrically erasable programmable randommemories (EEPROMs) or indeed flash memories, and the invention can beused with any other programmable memory that may become available in thefuture.

With reference to FIG. 2, there is shown a diagram representing theoperation of calibrating the FIG. 1 probe incorporating the programmablememory of the invention for the purpose of determining thecharacteristics of the Doppler transducer actually mounted in the probe,which characteristics are subsequently used for correcting the measuredflow speed of the fluid F circulating in the duct 10. For this purpose,a tank 60 is provided that is filled with a liquid, such as water, andthat has a well 60 a which is filled with water and in which the activeend of the probe 1 having the said transducers 4 and 5 is inserted. Thewell 60 a thus symbolizes the esophagus 13 of a human body. The tank 60has immersed therein a closed circuit 62 for circulating a fluid 63,such as water containing starch, so that the closed circuit 62symbolizes the aorta 10 in which blood is flowing, and the flow sectionS1 of the closed circuit 61 is calibrated, e.g. to a value S₁ that isclose to the flow section S of the blood vessel, so as to enable teststo be performed under conditions that are close to the genuine workingconditions of the probe 1 when in the human body. The speed at which theliquid 63 flows round the closed circuit 62 is determined by acting on apump or any other similar device for adjusting the flow speed of theliquid 63 in the closed circuit 62. The flow speed of said fluid is readby any appropriate flow measuring apparatus represented by 66. Duringthese tests at its manufacturing site, the probe is connected to thecontrol unit 8 which contains in particular the computation means 16such as a computer or a microcomputer. A screen 68 is generally alsoprovided on which various parameters are displayed together with theresults obtained.

The tests performed comprise fixing the flow speed of the liquid 63round the closed circuit 62 at various different values by means of theflow meter or spinner 64, thus making it possible to plot a calibrationcurve for the Doppler transducer 4.

An example of such a calibration curve is given in FIG. 3.

In FIG. 3:

-   -   the ordinate (reference flow rate) corresponds to reference        measurements as provided by the flow rate measuring apparatus 66        (FIG. 2); and    -   the abscissa (measured flow rate) corresponds to the        measurements performed by means of the probe, assuming that the        ultrasound beam is inclined at an angle θ of 60° (θ_(ideal)=60°)        and using the computation means 16 (FIG. 2) of the probe control        and computer unit 8 (FIG. 2).

The curve and the estimated angle and linear regression correlationrelating to the ultrasound beam are displayed on the screen 68 (FIG. 2).

The control unit 8 (FIG. 2) automatically saves the estimated angle(59.67° in this example) for the ultrasound beam in the memory of theprobe 50 (FIG. 2).

From the equation mentioned above:$V \approx {\frac{\Delta\quad F}{F_{emit}} \times \frac{C}{{2 \cdot {Cos}}\quad(\theta)}}$it can be shown that:$\theta_{{est}.} = {{ArcCos}\left( {\frac{{reference}\quad{flow}\quad{rate}}{{measured}\quad{flow}\quad{rate}} \times {Cos}\quad\left( \theta_{ideal} \right)} \right)}$$\frac{{reference}\quad{flow}\quad{rate}}{{measured}\quad{flow}\quad{rate}}$is obtained directly by the slope of the regression line through thepoints in FIG. 3.

1. A method for improving the accuracy with which the speed of bloodflowing in the aorta is measured by means of a signal emitted by aDoppler transducer, the method comprising inserting a Doppler effectintracorporeal probe into the esophagous of a subject, wherein: (a) aDoppler transducer is incorporated or integrated in the Doppler effectintracorporeal probe, the Doppler transducer is mounted on the probe toemit an ultrasound beam at an angle relative to the longitudinal axis ofthe probe, and the probe comprises a programmable memory; (b) theDoppler transducer is associated with the programmable memory, whereinthe programmable memory contains at least one signal correction dataitem for correcting the Doppler signal transmitted by the transducer toa transducer control and computer unit; and (c) the computer unitincorporates the signal correction data item in computing each speedmeasurement on the basis of each signal emitted by the Dopplertransducer, and computes the speed value taking account of the signalcorrection data item so as to provide a corrected measurement of thespeed of the blood, thereby improving the accuracy of the measurement.2. The method of claim 1, wherein: (a) the programmable memory alsocontains at least one sensitivity data item for informing the user of aloss of sensitivity to the Doppler signal; (b) the transducer controland computer unit verifies the sensitivity data item present in theprogrammable memory on each measurement of the signal transmitted by thetransducer in order to verify that the sensitivity actually obtained onthe signal transmitted by the transducer is not too far removed from thesensitivity value present in the programmable memory; and (c) in theevent of sensitivity going beyond a specified limit value, thetransducer control and computer unit issues to the user a signalindicative of a loss of sensitivity.
 3. The method of claim 2, whereinthe sensitivity data item is obtained during tests that are preferablyperformed at the manufacturing site while measuring the flow speed of afluid that is flowing at a known speed.
 4. The method of claim 2,wherein the sensitivity data item comprises at least one average of aplurality of sensitivity measurements obtained over a correspondingnumber of uses of the Doppler transducer, each sensitivity measurementresulting from the amplitude of the signal received from the transducer.5. The method of claim 2, wherein the transducer control and computerunit continuously computes the mean of a plurality of recentlycalculated sensitivity measurements and compares it with the sensitivitymean initially entered as sensitivity data into the programmable memory,and, beyond a certain difference relative to the initially programmedsensitivity measurement, issues a signal to the user indicative of aloss of sensitivity.
 6. The method of claim 1, wherein the signalcorrection data item is obtained from tests preferably performed at themanufacturing site while performing preliminary use tests on the Dopplertransducer in order to verify the reliability of its signal.
 7. Themethod of claim 1, wherein the signal correction data item comprises atleast the angle at which the Doppler beam is emitted by the Dopplertransducer relative to the axis of the probe, so that the speed valuetakes account of the real working angle of the beam from the Dopplertransducer.
 8. The method of claim 1, wherein, when the Dopplertransducer operates in combination with an additional transducer, formeasuring the diameter of the aorta, at least one sensitivity data itemconcerning the additional transducer is preferably also provided in theprogrammable memory, so as to verify its sensitivity over time, andlikewise issue a signal to the user in the event of a loss ofsensitivity.
 9. The method of claim 1, wherein the Doppler transducer ismounted on the probe to emit an ultrasound beam at an angle of 60degrees relative to the longitudinal axis of the probe.
 10. An apparatusfor improving the accuracy with which the speed of blood flowing in theaorta is measured by means of a signal emitted by a Doppler transducer,said apparatus comprising a programmable memory containing at least onesignal correction data item for correcting the Doppler signaltransmitted by the transducer to a transducer control and computer unit,wherein: (a) means are provided to enable the computer unit toincorporate the signal correction data item in computing each speedmeasurement on the basis of each signal emitted by the Dopplertransducer, and to compute the speed value taking account of the signalcorrection data item so as to provide a corrected measurement of thespeed of the blood, thereby improving its accuracy; and (b) the Dopplertransducer is incorporated or integrated in a Doppler effectintracorporeal probe that can be inserted into the esophagus, and theDoppler transducer is mounted on the probe to emit an ultrasound beam atan angle relative to the longitudinal axis of the probe, and wherein theprobe also comprises the programmable memory connected to the transducercontrol and computer unit, which memory is thus secured to the probe andis dedicated thereto.
 11. The apparatus of claim 10, wherein: (a) theprogrammable memory also contains at least one sensitivity data item forinforming the user of a loss of sensitivity to the Doppler signal; (b)the transducer control and computer unit verifies the sensitivity dataitem present in the programmable memory on each measurement of thesignal transmitted by the transducer in order to verify that thesensitivity actually obtained on the signal transmitted by thetransducer is not too far removed from the sensitivity value present inthe programmable memory; (c) signal-issuing means are provided; and (d)in the event of sensitivity going beyond a specified limit value, thetransducer control and computer unit issues a signal to the user, viathe signal-issuing means, to inform the user of a loss of sensitivity.12. The apparatus of claim 11, wherein the programmable memory containsa sensitivity data item is obtained during tests that are preferablyperformed at the manufacturing site while measuring the flow speed of afluid that is flowing at a known speed.
 13. The apparatus of claim 11,wherein the sensitivity data item comprises at least one average of aplurality of sensitivity measurements obtained over a correspondingnumber of uses of the Doppler transducer, each sensitivity measurementresulting from the amplitude of the signal received from the transducer.14. The apparatus of claim 11, wherein the transducer control andcomputer unit continuously computes the mean of a plurality of recentlycalculated sensitivity measurements and compares it with the sensitivitymean initially entered as sensitivity data into the programmable memory,and, beyond a certain difference relative to the initially programmedsensitivity measurement, issues a signal to the user, via thesignal-issuing means, to indicate a loss of sensitivity.
 15. Theapparatus of claim 10, wherein the programmable memory contains a signalcorrection data item is obtained from tests preferably performed at themanufacturing site while performing preliminary use tests on the Dopplertransducer in order to verify the reliability of its signal.
 16. Theapparatus of claim 10, wherein the signal correction data item comprisesat least the angle at which the Doppler beam is emitted by the Dopplertransducer relative to the axis of the probe, so that the speed valuetakes account of the real working angle of the beam from the Dopplertransducer as actually created on the probe.
 17. The apparatus of claim10, further comprising an additional transducer operating in combinationwith the Doppler transducer, said additional transducer for measuringthe diameter of the aorta, and said programmable memory preferablyfurther containing at least one sensitivity data item concerning theadditional transducer, so as to verify its sensitivity over time, andlikewise issue, via the signal-issuing means, a signal to the user inthe event of a loss of sensitivity.
 18. The apparatus of claim 10,wherein the Doppler transducer is mounted on the probe to emit anultrasound beam at an angle of 60 degrees relative to the longitudinalaxis of the probe.